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Acc. Chem. Res. 2001, 34, 319-330
Modular Chemistry: Secondary
Building Units as a Basis for
the Design of Highly Porous and
Robust Metal-Organic
Carboxylate Frameworks
MOHAMED EDDAOUDI,† DAVID B. MOLER,†
HAILIAN LI,† BANGLIN CHEN,†
THERESA M. REINEKE,†
MICHAEL O’KEEFFE,‡ AND OMAR M. YAGHI*,†
Materials Design and Discovery Group, Department of
Chemistry, University of Michigan, 930 North University
Avenue, Ann Arbor, Michigan 48109-1055, and Department
of Chemistry and Biochemistry, Arizona State University,
Box 871604, Tempe, Arizona 85287-1604
Received November 13, 2000
ABSTRACT
Secondary building units (SBUs) are molecular complexes and
cluster entities in which ligand coordination modes and metal
coordination environments can be utilized in the transformation
of these fragments into extended porous networks using polytopic
linkers (1,4-benzenedicarboxylate, 1,3,5,7-adamantanetetracarboxylate, etc.). Consideration of the geometric and chemical attributes
of the SBUs and linkers leads to prediction of the framework
topology, and in turn to the design and synthesis of a new class of
porous materials with robust structures and high porosity.
Bridging Molecular and Solid-State Chemistry
The construction of extended solids from molecular
building blocks is now of great interest due to the
advantages it offers for the design of materials. The use
of discrete molecular units in the assembly of extended
networks is an attractive synthetic approach since it
permits reactions to take place at or near room temper-
Mohamed Eddaoudi was born in Agadir, Morocco (1969). He received his B.S.
(1991) from the University of Ibnou Zohar with Honors, and M.S. (1992) and Ph.D.
(1996) from the University Denis Diderot Paris 7 with Tres Honorable avec
Felicitations du Jury. He has been a Faculty Research Associate with Professor
Yaghi since August 1997. His research focus is on the synthesis, characterization,
and inclusion/sorption chemistry of organic and inorganic porous materials.
David B. Moler was born in Ann Arbor, MI (1979). He is a junior student in the
Department of Chemical Engineering at the University of Michigan. His interests
in chemistry include molecular shapes, topology, and patterns.
Hailian Li was born in Jiangsu, China, in 1964. He received his B.S. (1987) and
M.S. (1990) in chemistry from Nanjing University, China. He served as a faculty
research associate with Professor Yaghi while at Arizona State University. At
present, he is a doctorate candidate at the Department of Chemistry of the
University of Michigan. His primary focus is on the synthesis of porous crystalline
materials with novel linkages.
Banglin Chen was born in Zhejiang, China (1965). He received his B.S. (1985)
and M.S. (1988) from Zhejiang University, and his Ph.D. (1999) from the National
University of Singapore. He has been a postdoctoral fellow in Professor Yaghi’s
group since August 1999. He has invested his efforts in the use of expanded
and decorated links for the synthesis of porous materials, in particular interwoven
structures.
10.1021/ar000034b CCC: $20.00
Published on Web 02/17/2001
 2001 American Chemical Society
ature, where the structural integrity of the building units
can be maintained throughout the reactionsan aspect
that allows for their use as modules in the assembly of
extended structures.1-18 Molecular modules can be designed to direct the formation of target structures as well
as to impart desired physical properties to solid-state
materials. In an effort to understand and fully develop this
area of chemistry, often termed modular chemistry, a large
number of synthetic approaches involving solution assembly of molecules have been pursued to achieve
functional mesoscopic phases, modified surfaces, and
designed crystals.1
We have focused on the study of crystalline materials
since the attainment of well-defined structures is intimately linked to an understanding of the design, synthesis,
and properties of materials. In particular, we have been
interested in the construction of porous materials due to
their immense impact on the global economy and the
fascinating prospects that open networks offer for building
complexity into molecular voids in which highly selective
inclusion and chemical transformations can be effected.
In an earlier Account,6 we showed how extended metalorganic frameworks (MOFs) can be designed, crystallized,
and fully characterized. Here, we present the next stage
in the development of MOFs as a new class of porous
materials: the recent progress in using the concept of
secondary building units (SBUs) for understanding and
predicting topologies of structures, and as synthetic
modules for the construction of robust frameworks with
permanent porosity. We also outline strategies for incorporating unprecedented arrays of coordinatively unsaturated open-metal (OM) sites in porous materials and for
the synthesis of modular interpenetrating and noninterpenetrating networks with optimal porosity.
* To whom correspondence should be addressed. E-mail: oyaghi@
umich.edu.
† University of Michigan.
‡ Arizona State University.
Theresa M. Reineke was born in St. Paul, MN, in 1972. She received her B.S.
degree from the University of Wisconsin-Eau Clair (1995), her M.S. from Arizona
State University (1998), and her Ph.D. in inorganic chemistry (2000) from the
University of Michigan, Ann Arbor, with Professor O. M. Yaghi. She is currently
a Postdoctoral Scholar at the California Institute of Technology with Professor
Mark Davis, studying nonviral gene delivery polymers.
Michael O’Keeffe was born in Bury St Edmunds, England (1934). He received his
B.Sc. (1954), Ph.D. (1958), and D.Sc (1976) from the University of Bristol. He is
Regents’ Professor of Chemistry at Arizona State University, where he has been
since 1963. His current research is particularly focused on studying beautiful
patterns found in chemistry and elsewhere.
Omar M. Yaghi was born in Amman, Jordan (1965). He received his B.S. in
chemistry from the State University of New York-Albany (1985) and his Ph.D.
from the University of Illinois-Urbana (1990) with Professor Walter G. Klemperer.
From 1990 to 1992, he was an NSF Postdoctoral Fellow at Harvard University
with Professor Richard H. Holm. He joined the faculty at Arizona State University
in 1992. He was awarded the ACS-Exxon Solid-State Chemistry Award in 1998.
In June 1999, he moved to the University of Michigan as a Professor of Chemistry,
establishing several research programs dealing with molecular and solid-state
chemistry, in particular the transformation of molecular organic and inorganic
building blocks to functional extended frameworks.
VOL. 34, NO. 4, 2001 / ACCOUNTS OF CHEMICAL RESEARCH
319
Modular Chemistry Eddaoudi et al.
FIGURE 1. Assembly of metal-organic frameworks (MOFs) by the copolymerization of metal ions with organic linkers to give (a) flexible
metal-bipyridine structures with expanded diamond topology and (b) rigid metal-carboxylate clusters that can be linked by benzene “struts”
to form rigid extended frameworks in which the M-O-C core (SBU) of each cluster acts as a large octahedron decorating a 6-connected
vertex in a cube. All hydrogen atoms have been omitted for clarity. (In (a), M, orange; C, gray, N, blue; in (b), M, purple; O, red; C, gray.
Structures were drawn using single-crystal X-ray diffraction data.)
Strategies for Construction of Rigid Porous
Frameworks
Although synthesis of open frameworks by assembly of
metal ions with di-, tri-, and poly-topic N-bound organic
linkers such as 4,4′-bipyridine (BPY) has produced many
cationic framework structures (Figure 1a), attempts to
evacuate/exchange guests within the pores almost invariably, with few exceptions,12,15 results in collapse of the host
framework. We have found that multidentate linkers such
as carboxylates allow for the formation of more rigid
frameworks due to their ability to aggregate metal ions
into M-O-C clusters that we refer to as secondary
building units (SBUs) (Figure 1b). The SBUs are sufficiently
rigid because the metal ions are locked into their positions
by the carboxylates; thus, instead of having one metal ion
320 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 34, NO. 4, 2001
at a network vertex (as is the case in M-BPY compounds),
now the SBUs serve as large rigid vertices that can be
joined by rigid organic links to produce extended frameworks of high structural stability. The frameworks are also
neutral, obviating the need for counterions in their
cavities.
To appreciate the impact of SBUs on pore size and
porosity of frameworks, it is instructive to compare two
strategies developed for the construction of highly porous
frameworks. (a) The use of long links that increase the
spacing between vertices in a net yields void space
proportional to the length of linker (Figure 1a). This means
that a bond is replaced by a sequence of bondssa process
we call expansion. Although in principle such expanded
structures provide for large pores, in practice they are
Modular Chemistry Eddaoudi et al.
often found to be highly interpenetrated and to have low
porosity. (b) In contrast, replacement of a vertex of a
framework net by a group of vertices, a process termed
decoration, results in open structures with high rigidity
and without a tendency to interpenetrate (Figure 1b). In
fact, when such networks are interpenetrating, optimal
pore volume may be achieved as described below. (The
replacement of the vertices of an N-connected net by a
group of N vertices is a special case of decoration which
we refer to as augmentation.24)
An increasing number of assembled frameworks have
as components polytopic groupings of linkers such as
1,3,5-benzenetribenzoate (BTB), which in themselves may
act to decorate a vertex in an assembly. Polytopic links
can thus be employed both to decorate and to expand a
net.14
It is worth noting that the sizes of rings (or pore size)
and voids in nets can be significantly increased with
decoration, augmentation, or a combination thereof. For
example, 4-rings in a simple cubic structure can be
augmented by octahedra (carboxylate carbon atoms in
Figure 1b, right) to produce a 5-connected structure with
8-ring pores and void space significantly larger than that
of the original structure.
From Molecular SBUs to Open Framework
Solids
Identification and Use of SBUs. Discrete di-, tri-, and
tetranuclear metal carboxylate clusters19-23 such as the
paddle-wheel copper acetate (D4h) and the basic zinc
acetate (Td) motifs were targeted as symmetrical modules
suited for polymerization reactions involving multidentate
carboxylate linkers. Such clusters can serve as SBUs in that
they have three components relevant to their polymerization into modular porous metal-carboxylate networks
(Figure 1b). (a) The M-O-C core structure is an SBU
whose shape is defined by those atoms representing
points of extension to other SBUs, and which are generally
separated only by links. Thus, such atoms define the
underlying geometry of the SBU; they are relevant to
predicting the overall topology of the modular network
(Figure 1b). (b) Potentially, each monocarboxylate ligand
in the molecular complex can be substituted with a di-,
tri-, or multicarboxylate in order to polymerize the SBU
into an extended network. However, the coordination
mode of each carboxylate ligand in the molecular complex
provides important geometric and conformational information that is critical to predicting the topology of the
resulting network. (c) In some clusters where terminal
ligands are present, their coordination sites may be
removed to allow the study of metal site reactivity: by
using weak ligands (such as methanol or ethanol) as
solvents in the synthesis, it is possible to produce extended
structures in which these ligands point toward the center
of the voids, making them susceptible to dissociation and
evacuation from the pores, thereby producing periodically
arranged OM sites.
FIGURE 2. Representative polytopic organic linkers. (Same coloring
scheme as in Figure 1.)
Solution Synthesis and Crystallization of Modular
Porous Solids. Generally, when 3-D extended solids linked
by strong chemical bonds are assembled under mild
temperature conditions, it is difficult to effect their
crystallization, and in general this remains a significant
obstacle to synthesis. However, we have developed pathways for synthesizing macrocrystalline modular solids. For
example, we have shown that copolymerization of SBUs
with the polytopic linkers 1,4-benzenedicarboxylate (BDC),
4,4′-azodibenzoate (ADB), 1,3,5-benzenetricarboxylate
(BTC), and 1,3,5,7-adamantanetetracarboxylate (ATC) (Figure 2) is an ideal pathway for obtaining crystalline
products. Typically, a solution of the acid form of a linker
and the appropriate simple metal salt (nitrate) is prepared
in the desired stoichiometry. The key step to obtaining
crystals is to slowly diffuse into the reaction mixture an
organic amine that deprotonates the acid and initiates the
assembly. Nucleation rates and the number of nucleation
sites can be controlled by controlling the rate of amine
diffusion, solvent polarity, and concentration gradients.
When the acid is poorly soluble, reactions are performed
above room temperature, up to 190 °C. In these cases,
inorganic hydroxides such as NaOH may be used (if
needed) instead of the organic amines to deprotonate the
acid.
Predicting Framework Topologies. A large number of
crystalline materials constructed from these SBUs and
linkers have been prepared and reported by our group.
Illustrative examples involving each SBU mentioned above
will be presented. In consideration of possible structures
that would form by the polymerization of the SBUs, we
rely on our thesis: in general only a small number of
simple, high-symmetry structures will be of overriding
general importance, and they would be expected to form
most commonly.24 We know of no systematic survey of the
occurrence of structural topologies, and when we say that
a particular topology is very common, we really mean that
it occurs very frequently in structures that we are likely
to have examined and to have analyzed their geometry.
Elaboration of this thesis has appeared in a recent tutorial
article.24 For the purposes of this presentation, a partial
list of structures that are most likely to form in the
assembly of certain symmetrical geometric shapes is
shown in Table 1 and represented in Figure 3.25 The
VOL. 34, NO. 4, 2001 / ACCOUNTS OF CHEMICAL RESEARCH 321
Modular Chemistry Eddaoudi et al.
Table 1. Partial List of Basic Nets with One or Two
Kinds of Vertex Figuresa
coordination
3
3
3
3,4
4
4
4,4
4
6
8
a
coordination figures
triangle
triangle
triangle
triangle
square
tetrahedron
square
square
octahedron
cube
triangle
triangle
triangle
square
square
tetrahedron
tetrahedron
square
octahedron
cube
net
SrSi2
ThSi2
63 honeycomb
Pt3O4
NbO
diamond (C)
cooperite (PtS)
44 square lattice
primitive cubic
body-centered cubic
Table 2. Comparison of Porosity in MOFsa
MOF MOF MOF MOF MOF MOF MOF
-2
-3
-4
-5
-6
-9
-11
pore diameter 7
8
14
12
(Å)
surface area
270
140
2900
(m2/g)
pore volume
0.094 0.038 0.612 1.04
(cm3/g)
4
8
7
127
560
0.099 0.035
0.20
a All parameters were obtained from gas sorption isotherm data.
Surface areas for MOF-4 and -6 are not included since these
frameworks did not adsorb gases.
For a more complete list of basic structures, see refs 24 and
25.
FIGURE 3. Common nets (see Table 1) for one and two kinds of
links. A segment of each net has been highlighted in orange for
clarity. (Structures were drawn using single-crystal X-ray diffraction
data.)
essence of this thesis is captured by discussion of the
topologies of extended porous structures prepared from
the SBUs discussed above. Unlike other MOFs, these SBUs
produce unprecedented porosity due to their rigid structure. A summary of porosity data obtained from gas
sorption isotherms is shown in Table 2 and discussed
further below.
Design of Periodic Molecular Imprints for
Sensing
Decoration and Expansion of Primitive Cubic Net. The
trinuclear SBUsknown in a molecular cluster21shas been
polymerized with BDC to form a 3-D porous network,
Zn3(BDC)3‚6CH3OH (MOF-3) (Figure 4).26 The central zinc
atom is octahedrally bound to carboxylate oxygens, and
each of the other zinc centers is coordinated to three such
oxygens in addition to two oxygens from terminal metha322 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 34, NO. 4, 2001
FIGURE 4. (a) Building unit in the crystal structure of Zn3(BDC)3‚
6CH3OH (MOF-3), in which each carboxylate carbon of four BDC
units and an oxygen from each of the remaining BDC links form. (b)
Octahedral (Oh) SBU, that assembles into (c) a primitive cubic-like
decorated diamond net topology. (Same coloring scheme as in Figure
1; structures were drawn using single-crystal X-ray diffraction data.)
nol ligands, where one methanol is bound more weakly
to zinc than the other (Figure 4a). In the crystal, two
additional methanol molecules are hydrogen bonded to
the methanol ligands, thus acting as guests to fill a 3-D
channel system of 8 Å cross section.
To determine the topology of MOF-3 based on the
guidelines presented above, each trinuclear unit can be
considered as an octahedral SBU (Figure 4b). Linking
these units with BDC produces the primitive cubic net
arrangement (Table 1, Figure 3i): the octahedral SBU
decorates the vertices that are then spaced (expanded)
from other such units by 2-connected benzene units (in
Modular Chemistry Eddaoudi et al.
FIGURE 5. (a) Building unit in the crystal structure of Zn2(BTC)(NO3)‚(C2H5OH)5(H2O) (MOF-4) having terminal ethanol and nitrate ligands
bound to opposite zinc centers. The carboxylate carbon atoms within each M-O-C cluster core and the 1,3,5-carbon atoms of each BTC
represent (b) two triangle SBUs (respectively shown in orange and gray), which assemble (c) into decorated Si net of SrSi2, to form (d) an
extended porous framework with open metal sites and large pores. (Same coloring scheme as in Figure 1; structures were drawn using
single-crystal X-ray diffraction data.)
the case of BDC having one carboxylate oxygen as part of
the SBU, the expansion is done with a benzyl unit) (Figure
4c).
We found that all methanol, including the ligands, can
be removed from the voids to give a porous network
(Table 2) having open zinc sites. It is interesting to note
that MOF-3 maintains its framework integrity, even in the
absence of methanol guests or ligandssan aspect that is
relevant to sensing and catalysis.27 In the following sections, we give examples that illustrate how the OM sites
can act as molecular imprints.
A Chiral Framework with Imprinted Crevices. A cubic
network with larger pores and with OM sites can be
achieved using the common dinuclear cluster D3h-Zn2(CO2)3.20 Copolymerization of BTC with zinc(II) yields a
building unit with a trigonal SBU geometry (Figure 5a) in
which the carbon atoms of the Zn-O-C cluster core are
the vertices of a trigonal SBU and three carbon atoms of
the benzene rings of BTC are the vertices of another
trigonal SBU (Figure 5b). This arrangement yields Zn2(BTC)(NO3)‚(C2H5OH)5(H2O) (MOF-4),28 which adopts one
of the most symmetric and frequently observed 3-connected topologies belonging to the set summarized in
Table 1, namely the chiral Si net of SrSi2 (Figure 3a).24,25
Here, each SBU and benzene unit alternately decorates
the Si vertices (Figure 5c).
Each building unit has a nitrate ion bound strongly to
one of the Zn centers and three ethanol molecules bound
weakly to the other Zn center. The crystal structure of
MOF-4 is cubic and contains 3-D channels of 14 Å
diameter filled with five ethanol (three act as ligands to
Zn and two are free guests filling the pores) and one water
molecule per formula unit. Using liquid and vapor sorption, it was shown that all ethanol and water can be
evacuated from the pores to produce pyramidal Zn sites
that are found to be highly selective to alcohol uptake.29
Guest competition experiments using 13C CPMAS NMR,
GC, and liquid isotherms showed that when evacuated
MOF-4 is exposed to an equimolar mixture of CH3CN and
CH3OH, only CH3OH is allowed into the pores. Similar
experiments involving larger molecules produced similar
affinity of the pores to alcohols. This has led us to
conclude that the exposed zinc sites are part of a crevice
left behind by the liberated ethanol, where a geometric
and electronic imprint (specific host-guest hydrogenbonding interactions) for alcohols has been registered at
the binding sites (Figure 5d).29
Design and Crystal Structure of Materials with
Open Metal Sites
Decorated Square Grids. The paddle-wheel cluster is
known in many complexes;19 it has a square geometry
(Figure 6a), which when linked with benzene, a linear
ditopic “strut”, produces Zn(BDC)(H2O)‚(DMF) (MOF-2);
DMF ) N,N′-dimethylformamide) having a layered structure.32 Here, Zn2(CO2)4 square SBUs decorate the intersection of a 44 planar grid, and the benzene units expand
the links between vertices to produce the 4.82 tiling pattern
(Figure 6b). The axial positions on Zn are occupied by
water ligands which serve to hold together the sheets by
mutual hydrogen bonding to the carboxylate oxygen on
VOL. 34, NO. 4, 2001 / ACCOUNTS OF CHEMICAL RESEARCH 323
Modular Chemistry Eddaoudi et al.
FIGURE 6. (a) Paddle-wheel cluster, a square SBU, which is the
building unit in the crystal structure of Zn(BDC)(H2O)‚(DMF) (MOF2), which forms (b) a decorated square grid and (c) an open
framework with DMF occupying the pores. (Zn, yellow; O, red; N,
green; C, gray. Structures were drawn using single-crystal X-ray
diffraction data.)
an adjacent sheet. The sheets stack in registry to allow
the formation of 1-D channels where DMF guests reside
(Figure 6c). The DMF and water can be thermally removed
to allow each metal to bond to a carboxylate oxygen of
an adjacent sheet, thereby linking the sheets in the third
dimension and leading to a quasi 3-D network with open
channels. The porosity of this material was examined and
found to exhibit zeolite-like type I gas sorption isotherms
(Table 2).32
Porous Crystals with Open Metal Sites. It is wellknown that when the axial ligands in the paddle-wheel
structure have been dissociated, as in MOF-2, the metal
sites thus produced bind to a Lewis base atom on a
neighboring unit to give polymeric structures having no
OM sites.33 Our strategy to prevent coupling of these units
and to achieve an MOF with accessible OM sites relies
on using the square SBU with a tetrahedral linker. Using
the organic adamantane cluster as a second SBU, we
expected that the combination of a square (Figure 7a,b)
and a tetrahedron would polymerize to form the PtS
topology, as indicated in Table 1 (Figure 3g). Here, each
square Pt atom and tetrahedral S atom in PtS would be
replaced by an inorganic square and an organic tetrahedral SBU, respectively, to give a decorated form of PtS
(Figure 7c). Indeed, the success of this approach has been
324 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 34, NO. 4, 2001
demonstrated by performing reactions that give the copper carboxylate square motif: addition of Cu(NO3)2‚
2.5H2O to 1,3,5,7-adamantanetetracarboxylic acid (H4ATC)
in basic aqueous solution at 190 °C yields green crystals
of Cu2(ATC)‚6H2O (MOF-11) having the desired geometry
in which self-aggregation of SBUs is prevented since the
labile ligands point toward the centers of voids (Figure
7d).34 Each square has two axial water ligands (Cu-OH2
) 2.148(3) Å), one bound to each of its copper(II) centers.
These point away from the Cu-O-C framework into a
3-D channel system of 6.0-6.5 Å diameter, where four
additional water guests per square unit reside. Estimates
based on van der Waals radii showed that water ligands
and guests occupy approximately 50% of the crystal
volume. All water guests and ligands were liberated to give
an anhydrous framework with a thermal stability range
120-260 °C. Furthermore, the rigid and porous nature of
the anhydrous framework was evident upon measurement
of its gas sorption isotherms, where fully reversible type I
behavior was observed for N2(g) and Ar(g)sunequivocally
confirming the existence of permanent porosity in anhydrous MOF-11 (Table 2).
The crystal structure of the anhydrous material (Figure
7d) showed that the originally monoclinic framework
relaxes to the tetragonal symmetry of the ideal PtS net
with negligible change in volume (Table 3). The absence
of water ligands on copper is evident from the shorter
distance observed for Cu-OCO and a significant shortening of the Cu-Cu distance upon liberation of watersthe
latter (2.490 Å) being the shortest distance known for
copper(II) carboxylate compounds. Magnetic susceptibility
data obtained for the as-synthesized MOF-11 follow the
expected behavior typically observed for antiferromagnetic
coupling in the copper(II) acetate dimer (2J ) -280 cm-1);
however, the anhydrous material showed increased coupling (2J ) -444 cm-1), as expected for the observed
shortening of the Cu-O distance found in the X-ray crystal
diffraction analysis.
The full characterization of the OM sites in MOF-11
paves the way for their use in sensors, in catalysis, and as
functionalization sites for mounting molecular moieties
into the pores. An example of the potential use of OM
sites in sensing is presented in the section that follows.
Luminescent Porous Frameworks. To examine the
viability of MOFs with OM sites in sensing small molecules, we initiated a project aimed at exploring the
chemistry of frameworks with lanthanide metal ions. It
was shown that the compound Tb2(BDC)3‚(H2O)4 (MOF6), which has an extended nonporous structure, can be
converted to a porous network, Tb2(BDC)3, by thermally
liberating the water ligands (Table 2).30 The dehydrated
form has extended 1-D channels and the same framework
structure as that of the as-synthesized solid, as shown by
X-ray powder diffraction. Water sorption isotherm data
proved that Tb2(BDC)3 has permanent porosity and accessible Tb(III) sites. Luminescence lifetime measurements confirmed that when water is resorbed or ammonia
is sorbed into the pores, they bind to Tb, giving distinctly
different decay luminescence lifetime constants. Further
Modular Chemistry Eddaoudi et al.
FIGURE 7. (a) Paddle-wheel Cu2(OCO)4 SBU (Cu, blue; C, green; O, red) of square geometry (green) and (b) adamantane SBU (C, green) of
tetrahedral geometry (red). These building blocks assemble to adopt the PtS net with its vertices occupied by the clusters to form (c) a
decorated PtS framework, where (d) open metal sites point into the pores (same color scheme as in (a) and (b)). (Structures were drawn
using single-crystal X-ray diffraction data.)
Table 3. Crystallographic Data and Important Interatomic Distances for MOF-11
crystals,
chemical formula
crystal system,
space group
as-synthesized Cu2(ATC)‚6H2O
monoclinic, C2/c
dehydrated Cu2(ATC)
tetragonal, P42/mmc
a
unit cell:
a, b, c (Å);
R, β, γ (°)
a)
b ) 8.5732
c ) 14.3359
R ) 93.92
β ) 87.08
γ ) 98.27
a ) 8.4671(2)
b ) 8.4671(1)
c ) 14.444(1)
8.5732a
Cu-OCO
distance (Å)
Cu-Cu
distance
(Å)
V (Å3)
Z
R1
(I > 2σ(I))
1040.4(1)
2a
0.0433
1.976(2)
1.989(2)
1.964(2)
2.622(3)
1035.5(1)
2
0.0353
1.935(2)
2.490(3)
Primitive cell. The conventional C-centered cell has a ) 12.98658(2), b ) 11.2200(1), and c ) 11.3359(2) Å, β ) 93.857(1)°, Z ) 4.
work along these lines has produced another framework
with larger pores that potentially may exhibit more diverse
luminescent properties.31
Beyond Zeolites: MOFs with High Porosity and
Stability
MOF-5. The basic zinc acetate structure22 can be extended
into a 3-D network with BDC to give Zn4O(BDC)3‚(DMF)8(C6H5Cl) (MOF-5), a simple cubic 5-connected net, as
indicated in Table 1 (Figure 8a,b): the nodes (vertices) of
the cubic net are replaced by octahedral SBUs, and the
links (edges) of the net are replaced by finite rods of BDC
atoms.35 The pores are a 3-D channel system of 8 Å
aperture and 12 Å cross section filled with eight DMF
SBUs and one chlorobenzene SBU (Figure 8c). The guests
can be easily exchanged or/and evacuated from the pores
without loss of the framework integrity.
To evaluate the pore volume and the apparent surface
area of this framework (Table 2), the gas and vapor
sorption isotherms for the desolvated sample were measured. N2(g) sorption revealed a reversible type I isotherm
(Figure 9). The same sorption behavior was observed for
Ar(g) and organic vapors such as CH2Cl2, CHCl3, CCl4, C6H6,
and C6H12. Similar to those of most zeolites, the isotherms
are reversible and show no hysteresis upon desorption of
gases from the pores. The apparent Langmuir surface area
was estimated at 2900 m2/g. Typically for crystalline
zeolites, which generally have higher molar mass than
Zn4O(BDC)3, surface areas ranging up to 500 m2/g are
found.
Crystal Structure of MOF-5 with Completely Empty
Channels. At the outset of this work, the porosity of MOFs
was unexamined, leaving doubts as to whether such
frameworks were viable as porous materialssa question
of considerable interest. Starting with MOF-2, we showed
VOL. 34, NO. 4, 2001 / ACCOUNTS OF CHEMICAL RESEARCH 325
Modular Chemistry Eddaoudi et al.
FIGURE 8. (a) Building unit present in crystals of Zn4O(BDC)3‚(DMF)8(C6H5Cl) (MOF-5), where the carboxylate carbon atoms are in an
octahedral geometry decorating (b) a primitive cubic lattice, and form
(c) the most porous crystals known to date. (Same coloring scheme
as in Figure 1; structures were drawn using single-crystal X-ray
diffraction data.)
FIGURE 9. Typical gas sorption isotherms for MOFs, shown here
for MOF-5.
that modular MOFs prepared from metal carboxylates can
be constructed with sufficient rigidity and structurally
stability to support permanent porosity in the absence of
guests. Since then, we have shown numerous other
examples of MOFs that exhibit zeolite-like gas sorption
isotherms.
Much of the early work has been subject to one or more
of the following criticisms. (a) The crystal structures are
326 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 34, NO. 4, 2001
unreliable because disorder of the guests leads to large R
values. (b) The guests cannot be completely removed from
the pores without collapse of the framework. (c) Crystal
structures of empty frameworks with significant porosity
have not been reported. In this section we show that it is
possible to obtain the crystal structure of a highly porous
framework, where the guests have been removed from the
channels and the crystals placed under vacuum.
In an influential paper, Harlow36 identifies several
classes of crystal structures, two of which concern us here.
(a) Quality structures: for an organometallic structure it
is stated that if the hydrogen atoms can be refined with
C-H distances between 0.85 and 1.05 Å and isotropic
atom displacement parameter, B, less than 6 Å2, “the data
must be of high quality and the model must be correct”.
(b) At the other extreme come junk structures. These have
large R values; it is suggested that for R > 0.15, “there are
no useful lessons to be learned from them”. For those
interested in constructing highly porous metal-organic
networks, the great majority of such structures published
to date fall more nearly into the second category, and
there is a continuing debate as to their reliability.
In the cubic porous material, Zn4O(BDC)3 (MOF-5),
discussed above, the DMF and chlorobenzene guests in
the cavities of the as-synthesized form of MOF-5 are
disordered and could not be refined in the original
structure determination, and the resulting crystal structure
had R1 ) 0.11 (Table 4). Direct evidence for the striking
stability of the framework was obtained by evacuating the
pores and performing a structure determination on the
desolvated crystals which showed that the cell parameters
were virtually unchanged. In addition, heating the fully
desolvated crystals in air at 300 °C for 24 h had no impact
on either their morphology or their crystallinity, as
evidenced by another X-ray single-crystal diffraction study.
Here, again, the cell parameters obtained were virtually
unaltered from those found for the unheated desolvated
crystals. We subsequently found that crystal integrity is
maintained even when the crystal is placed in vacuo. A
structure determination, which refined to R ) 0.019 (see
Tables 4 and 5), was carried out on the crystal placed
under vacuum. The H atoms of the BDC groups refined
to give a carbon-hydrogen bond length of 0.93(3) Å and
B ) 5.5 Å2. The maximum peak in the final difference map
was 0.2 e- Å-3, showing that the channels in the structure
were really empty.
It is worth noting that by Harlow’s criteria we have
gone from close to a “junk structure” to a “quality
structure”. It is therefore of considerable interest to
compare the two structures, as is done in Tables 4 and 5.
The structures are generally in excellent agreement but
have esd’s about 10 times larger for the solvated sample.
This translates into errors for non-hydrogen bond lengths
being as much as 0.03 Å for the solvated structure
compared to <0.004 Å for the evacuated sample. However,
apart from the difference in esd’s, the structures are
essentially identical.
It is important to recognize that the evacuated structure
of MOF-5 is, we believe, the most porous crystal structure
Modular Chemistry Eddaoudi et al.
Table 4. Crystal Data for MOF-5
T (K)
space group
a (Å)
Z
V (Å3)
F (g/cm3)
R, Rw
max./min. (e-/Å3)
Zn4O(BDC)3‚
(DMF)8(C6H5Cl)
as synthesized
fully desolvated
300 °C for 24 h in air
vacuum
213 ( 2
Fm3
hm
25.6690(3)
8
16 913.2(3)
1.2
0.11, 0.31
1.56/-0.45
169 ( 2
Fm3
hm
25.8849(3)
8
17 343.6(3)
0.59
0.023, 0.026
0.25/-0.17
149 ( 2
Fm3
hm
25.8496(3)
8
17 272.7(3)
0.59
165 ( 2
Fm3
hm
25.8556(3)
8
17 284.8(3)
0.59
0.019/0.024
0.20/-0.30
Zn4O(BDC)3
Table 5. Comparison of Atomic Coordinates for
As-Synthesized (Solvated) and Evacuated (Vacuum)
MOF-5
atom
compound
x
y
z
B (Å2)
Zn
vacuum
solvated
vacuum
solvated
vacuum
solvated
vacuum
solvated
vacuum
solvated
vacuum
solvated
vacuum
solvated
0.293477(7)
0.2935(1)
0.11146(10)
0.1105(7)
0.05386(10)
0.0538(7)
0.21751(6)
0.2169(4)
1/4
1/4
0.21937(4)
0.2182(3)
0.1956(7)
0.1941
x
x
1/4
1/4
1/4
1/4
x
x
1/4
1/4
x
x
x
1/4
1/4
1/4
1/4
0.47343(8)
0.4737(6)
1/4
1/4
0.36611(5)
0.3661(4)
0.45520(1)
0.4548
1.714(2)
4.0(1)
2.53(3)
6.9(2)
2.75(3)
7.7(6)
4.11(3)
9.7(6)
3.08(2)
3.3(4)
1.63(1)
7.6(3)
5.5(7)
11.7
C1
C2
C3
O1
O2
H1
x
x
ever determined; certainly it is one of the least dense (of
crystalline materials stable at room temperature and
pressure, metallic lithium appears to be the only one less
dense). The density of the evacuated crystal is 0.59 Mg
m-3, compared with 1.14 Mg m-3 for the solvated crystal.
This study confirmed for the first time that, with a stable
and rigid framework such as that of MOF-5, permanent
porosity exists even in the absence of guests and that the
structure of the evacuated framework can be determined
with high accuracy by single-crystal diffraction techniques.
It also showed that, although crystal structures of open
structures with disordered guest molecules and other
species have high R values, the positions of the framework
atoms can be determined reliablysa point of some dispute
in the past.
The Role of SBUs in Catenation: Achieving
Optimal Porosity
Interpenetration. The Tb2C4O8 core units (already known
in molecular clusters)23 can be employed as an octahedral
SBU (Figure 10 a,b). This SBU is copolymerized with the
long dicarboxylate linker ADB to produce Tb2(ADB)3‚
[(CH3)2SO]20 (MOF-9), in which an additional bidentate
carboxylate is bound to each Tb; however, the octahedral
topology of the SBU is still preserved. Crystals of MOF-9
have a doubly interpenetrating structure with each framework having an idealized simple cubic 6-connected net
(Figure 10c).37 Despite the presence of two interpenetrating networks, at least 20 DMSO guest molecules per SBU
occupy the pores (Table 2), or a volume representing 71%
of the crystal volume, a value greater than previously
observed for interpenetrating structures. To explain this
FIGURE 10. Crystal structure of MOF-9 (shown at 45° from the ab
plane) constructed from (a) approximately octahedral Tb2(ADB)3(DMSO)4 building units (Tb, orange; O, red; C, gray; N, blue) with the
DMSO ligands (two on each Tb atom) not shown. A representation
of this SBU is shown in (b) as an octahedron linked to ADB (space
filling). This gives (c) two interpenetrating networks (colored
separately in blue and orange) with large void space.
unusually large void space in a maximally interpenetrating
network, we considered a simple geometric argument
involving spherical SBUs of diameter d that form a
primitive cubic network with linkers of length l. The cubic
cell edge is a ) d + l. If the van der Waals diameter (δ) of
the atom joining the SBU and linker is considered, then
for n frameworks to interpenetrate with centers of the
SBUs aligned along a body diagonal, n(d + δ) e 31/2a; thus,
ne 31/2(d + l)/(d + δ).
A composite plot (Figure 11) of n as a function of d
and l and their relationship to the free volume is used to
gain insight into interpenetrating structures. The plot
clearly shows that the level of interpenetration is predominantly determined by the length of linkers, l, while
the free volume is largely influenced by the size of the
VOL. 34, NO. 4, 2001 / ACCOUNTS OF CHEMICAL RESEARCH 327
Modular Chemistry Eddaoudi et al.
FIGURE 11. A d-l-n-v plot for a cubic system having spherical SBUs and cylinder-shaped rods: n is plotted (total number of frameworks
in a structure) as a function of d (diameter of an SBU) and l (length of linker), with the corresponding free volume expressed as the percent
of crystal volume shown in decreasing shades of darkness (darkest, <20, to lightest, >80%).
SBUs, d. Significantly, for structures having a small degree
of interpenetration (n ) 2 or 3 in the example here), an
optimum amount of available free volume can be achieved.
Also, due to the use of bulky SBUs, structures having large
free volume are possible at an even higher level of
interpentration (n g 4). Furthermore, the free volume
becomes smaller at high degrees of interpenetration (with
relatively small SBUs) because now much of the available
space is taken up by the linkers.
The case of MOF-9 can be considered here although it
is not strictly cubic: we find that d ) 9.65 Å and l ) 11.88
Å, which lies in the region of n ) 2 with void volume of
60-80%. Note that if d were shorter by only 2 Å, then a
structure having three interpenetrating frameworks with
less free volume (40%) would be expected (Figure 11).
Thus, the Tb-O-C cluster served as an optimal SBU for
ADB, allowing MOF-9 to just miss having a third interpenetrating framework. It should be noted that although
large porosity can be achieved for a given structure, the
pore aperture is completely dependent on the diameter
of the SBU. Nevertheless, it should be possible to access
apertures in the micro- and mesoporous regimes using
ordinary SBUs and organic links.
Interweaving. The structures most commonly found
interpenetrating are based on the SrSi2, diamond, and
simple cubic nets. In these cases a net and its interpenetrating partner are maximally displaced from the periodic
surfaces (the G, D, and P minimal surfaces,38 respectively)
that separate them. However, the vertices of the most
symmetrical (3,4)-connected net, that named for Pt3O424,25
(cf. Table 1), lie on the cubic P surface. In the structure
with a second interpenetrating net (displaced from the
328 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 34, NO. 4, 2001
first by 1/2,1/2,1/2) the 4-connected vertices, all on the same
surface, avoid each other, but the 3-connected vertices
superimpose. However, the 3-connected vertices can be
symmetrically displaced from the surface to produce an
interwoven structure. We refer to the resulting structure
as “interwoven” rather than “interpenetraing” as the two
nets reinforce each other while the cavities of each net
superimpose and remain empty. We have recently14
exploited this principle to prepare two interwoven Pt3O4
nets in which the square SBU is again the Cu2 paddlewheel and the tritopic linker is 1,3,5-benzenetribenzoate
(BTB). The resulting cubic framework with composition
Cu2(BTB)3‚(H2O)2 is remarkably robust and has the largest
accessible pores (16.4 Å diameter) yet found in a material
of this kind.
Concluding Remarks
It has now been demonstrated that a new class of
crystalline porous materials (MOFs) have been synthesized
and fully characterized. The ability to design their structures has allowed the synthesis of compositions with
designed pore structure (size, shape, and function) and
porositysaspects well beyond what has been achieved
with the most porous zeolites and related oxide molecular
sieves. The successful use of secondary building units in
the formation of certain predicted structures and its
impact on identifying networks with optimal porosity is
evidence that chemists are at a stage where the primary
backbone of extended structures (linked by strong bonds)
can be designed.
Modular Chemistry Eddaoudi et al.
Establishing porosity in MOFs as illustrated by the
successful single-crystal structure analysis of fully evacuated frameworks and gas sorption isotherms constitutes
an important step in extending the concepts of design
toward addressing outstanding challenges. Namely, (a) the
important one of functionalizing the pores with useful
molecular moieties that introduce weaker and reversible
interactions (complex function) into such open frameworks, (b) the assembly and use of less symmetric SBUs,
including chiral ones, and (c) the design of dimensionally
larger SBUs that still have a small number (3-6) of points
of connection allowing them to be incorporated into open
frameworks. Current work in this group is focused on
addressing these challenges and on pursuing the applications of these materials in catalysis, gas and liquid
separations, methane and hydrogen storage, and luminescence-based sensors.
The authors acknowledge the contributions of our colleagues
cited in the references and support from the National Science
Foundation (DMR-9980469 and DMR-9804817) and the Department of Energy.
References
(1) Modular Chemistry; Michl, J., Ed.; Kluwer Academic Publishers:
Dordrecht, 1995, and references therein.
(2) Bowes, C. L.; Ozin, G. A. Self-Assembling Frameworks: Beyond
Microporous Oxides. Adv. Mater. 1996, 8, 13-28.
(3) Cheetham, A. K.; Férey, G.; Loiseau, T. Open-Framework Inorganic
Materials. Angew. Chem., Int. Ed. 1999, 28, 3268-3292.
(4) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Organic-Inorganic
Hybrid Materials: From “Simple” Coordination Polymers to
Organodiamine-Templated Molybdenum Oxides. Angew. Chem.,
Int. Ed. 1999, 38, 2638-2684.
(5) Stein, A.; Keller, S. W.; Mallouk, T. E. Turning Down the Heat:
Design and Mechanism in Solid-State Synthesis. Science 1993,
259, 1558-1564.
(6) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Synthetic
Strategies, Structure Patterns, and Emerging Properties in the
Chemistry of Modular Porous Solids. Acc. Chem. Res. 1998, 31,
474-484.
(7) Hennigar, T. L.; MacQuarrie, D. C.; Losier, P.; Rogers, R. D.;
Zaworotko, M. J. Supramolecular Isomerism in Coordination
Polymers: Conformational Freedom of Ligands in [Co(NO3)2(1,2bis(4-pyridyl)ethane)1.5]n. Angew. Chem., Int. Ed. Engl. 1997, 36,
972-973.
(8) Batten, S. R.; Robson, R. Interpenetrating Nets: Ordered, Periodic
Entanglement. Angew. Chem., Int. Ed. 1998, 37, 1460-1494.
(9) Kitagawa, S.; Kondo, M. Functional Micropore Chemistry of
Crystalline Metal Complex-Assembled Compounds. Bull. Chem.
Soc. Jpn. 1998, 71, 1739-1753.
(10) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W.-S.; Withersby, M. A.; Schröder, M. Inorganic Crystal Engineering Using
Self-Assembly of Tailored Building-Blocks. Coord. Chem. Rev.
1999, 183, 117-138.
(11) Lu, J.; Paliwala, T.; Lim, S. C.; Yu, C.; Niu, T.; Jacobson, A. J.
Coordination Polymers of Co(NCS)2 with Pyrazine and 4,4′Bipyridine: Syntheses and Structures. Inorg. Chem. 1997, 36,
923-929.
(12) Biradha, K.; Hongo, Yoshito, H.; Fujita, M. Open Square-Grid
Coordination Polymers of the Dimensions 20 × 20 Å: Remarkably
Stable and Crystalline Solids Even after Guest Removal Angew.
Chem., Int. Ed. 2000, 39, 3843-3845.
(13) Xiong, R.-G.; Wilson, S. R.; Lin, W. Bis(Isonicotinato)Iron(II): A
Rare, Neutral Three-Dimensional Iron Coordination Polymer. J.
Chem. Soc., Dalton Commun. 1998, 4089-4090.
(14) Chen, L.; Eddaoudi, M.; Hyde, S. T.; O’Keeffe, M.; Yaghi, O. M.
Interwoven Metal-Organic Framework on a Periodic Minimal
Surface with Extra-Large Pores. Science 2001, 291, 1021-1023.
(15) Kepert, C. J.; Rosseinsky, M. J. Zeolite-Like Crystal Structure of
an Empty Microporous Molecular Framework. Chem. Commun.
1999, 375-376.
(16) Goodgame, D. M. L.; Grachvogel, D. A.; Williams, D. J. A New
Type of Metal-Organic Large-Pore Zeotype. Angew. Chem., Int.
Ed. 1999, 38, 153-156.
(17) Carlucci, L.; Ciani, G.; Proserpio, D. M. Interpenetrated and
Noninterpenetrated Three-Dimensional Networks in the Polymeric
Species Ag(tta) and 2 Ag(tta)‚AgNO3(tta)tetrazolate): The First
Examples of the µ4-η1:η1:η1:η1 Bonding Mode for Tetrazolate.
Angew. Chem., Int. Ed. 1999, 38, 3488-3492.
(18) Kiang, Y.-H.; Gardner, G. B.; Lee, S.; Xu, Z.; Lobkovsky, E. B.
Variable Pore Size, Variable Chemical Functionality, and an
Example of Reactivity within Porous Phenylacetylene Silver Salts.
J. Am. Chem. Soc. 1999, 121, 8204-8215.
(19) Clegg, W.; Little, I. R.; Straughan, B. P. Zinc Carboxylate Complexes: Structural Characterisation of Some Binuclear and Linear
Trinuclear Complexes. J. Chem. Soc., Dalton Trans. 1986, 12831288.
(20) Malik, M. A.; Motevalli, M.; O’Brien, P. Structural Diversity in the
Carbamato Chemistry of Zinc: X-ray Single-Crystal Structures of
[(Me2NCH2)2 Zn(O2CN(C2H5)2)2] and [C5H5NZn2Me(O2CN(C2H5)2)3].
Inorg. Chem. 1995, 34, 6223-6225.
(21) Clegg, W.; Little, I. R.; Straughan, B. P. Preparation and Crystal
Structure of a Carboxylate-bridged Linear Trinuclear Zinc(II)
Complex. J. Chem. Soc., Chem. Commun. 1985, 73-74.
(22) Clegg, W.; Harbron, D. R.; Homan, C. D.; Hunt, P. A.; Little, I. R.;
Straughan, B. P. Crystal Structures of Three Basic Zinc Carboxylates Together with Infrared and FAB Mass Spectrometry Studies
in Solution. Inorganica Chimica Acta 1991, 186, 51-60.
(23) Kuz’mina, N. P.; Martynenko, L. I.; Tu, Z. A.; Nguet, C. T.; Troyanov,
S. I.; Rykov, A. N.; Korenev, Y. M. Rare-Earth(III) Pivalates. Russ.
J. Inorg. Chem. 1994, 39, 512-520.
(24) O’Keeffe, M.; Eddaoudi, M.; Li, H.; Reineke, T. M.; Yaghi, O. M.
Frameworks for Extended Solids: Geometrical design Principles.
J. Solid State Chem. 2000, 152, 3-20.
(25) O’Keeffe, M.; Hyde, B. G. Crystal Structures. I. Patterns and
symmetry; Mineralogical Society of America: Washington, DC,
1996.
(26) Li, H.; Davis, C. E.; Groy, T. L.; Kelley, D. G.; Yaghi, O. M.
Coordinatively unsaturated metal centers in the extended porous
framework of Zn3(BDC)3‚6CH3OH (BDC ) 1,4-benzenedicarboxylate). J. Am. Chem. Soc. 1998, 120, 2186-2187.
(27) Eddaoudi, M.; Li, H.; Reineke, T.; Fehr, M.; Kelley, D.; Groy, T. L.;
Yaghi, O. M. Design and synthesis of metal-carboxylate frameworks with permanent microporosity. Top. Catal. 1999, 9, 105111.
(28) Yaghi, O. M.; Davis, C. E.; Li, G.; Li, H. Selective guest binding by
tailored channels in a 3-D proous zinc(II)-benzenetricarboxylate
network. J. Am. Chem. Soc. 1997, 119, 2861-2868.
(29) Eddaoudi, M.; Li, H.; Yaghi, O. M. Highly porous and stable metalorganic frameworks: structure design and sorption properties.
J. Am. Chem. Soc. 2000, 122, 1391-1397.
(30) Reineke, T. M.; Eddaoudi, M.; Fehr, M.; Kelley, D.; Yaghi, O. M.
From condensed lanthanide coordination solids to microporous
frameworks having accessible metal sites. J. Am. Chem. Soc.
1999, 121, 1651-1657.
(31) Reineke, T. M.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. A
microporous lanthanide-organic framework. Angew. Chem., Int.
Ed. 1999, 38, 2590-2594.
(32) Li, H.; Eddaoudi, M.; Groy, T. L.; Yaghi, O. M. Establishing
microporosity in open metal-organic frameworks: gas sorption
isotherms for Zn(BDC) (BDC ) 1,4-benzenedicarboxylate). J. Am.
Chem. Soc. 1998, 120, 8571-8572.
(33) Agterberg, F. P. W.; Provó Kluit, A. J.; Driessen, W. L.; Oevering,
H.; Buijs, W.; Lakin, M. T.; Speck, A. L.; Reedijk, J. Dinuclear
paddle-wheel copper(II) carboxylates in the catalytic oxidatio of
carboxylic acids. Unusual polymeric chains found in the singlecrystal X-ray structures of [tetrakis(µ-1-phenylcyclopropane-1carboxylato-0,0′)bis(ethanol-0)dicopper(II)] and catena-poly[[bis(µdiphenylacetato-0:0′)dicopper](µ3-diphenylacetato-1-0:2-0′:1′-0′)(µ3-diphenylacetato-1-0:2-0′:2′-0′)]. Inorg. Chem. 1997, 36, 43214328.
(34) Chen, B.; Eddaoudi, M.; Reineke, T. M.; Kampf, J. W.; O’Keeffe,
M.; Yaghi, O. M. Cu2(ATC)‚6H2O: Design of open metal sites in
porous metal-organic crystals (ATC: 1,3,5,7-adamantane tetracarboxylate). J. Am. Chem. Soc. 2000, 122, 11559-11560.
(35) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Design and
synthesis of an exceptionally stable and highly porous metalorganic framework. Nature 1999, 402, 276-279.
(36) Harlow, R. L. Troublesome crystal structures: prevention, detection, and resolution. J. Res. Natl. Inst. Stand. Technol. 1996, 101,
327-339.
VOL. 34, NO. 4, 2001 / ACCOUNTS OF CHEMICAL RESEARCH 329
Modular Chemistry Eddaoudi et al.
(37) Reineke, T. M.; Eddaoudi, M.; Moler, D.; O’Keeffe, M.; Yaghi, O.
M. Large free volume in maximally interpenetrating networks:
the role of secondary building units exemplified by Tb2(ADB)3[(CH3)2SO]4‚16[(CH3)2SO]. J. Am. Chem. Soc. 2000, 122, 4843-4844.
330 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 34, NO. 4, 2001
(38) Hyde, S. T.; Ninham, B.; Andersson, S.; Blum, Z.; Landh, T.;
Larsson, K.; Lidin, S. The Language of Shape; Elsevier: Amsterdam. 1997.
AR000034B